FIGS. 16A and 16B show an example of a conventional surface inspection apparatus (as disclosed, for example, in Japanese patent publication No. 52-21387).
In FIGS. 16A and 16B, on the upper left side of an inspected object or a sheet material 7 moved in the direction of an arrow 8 by means of a drive mechanism (not shown) is disposed an optical scanner 1. The optical scanner 1 is moved in the perpendicular direction 9 to the moving direction 8 of the sheet material 7 and in the direction parallel to the sheet material 7 by means of a mover mechanism (not shown), and irradiates laser light (hereinafter "light") towards the surface of the sheet material 7. On the upper right side of the sheet material 7, a convex lens 2 which receives reflected light from the surface of the sheet 7 and sends transmitted light therethrough to a spatial filter 4 above the convex lens 2, is arranged in the direction perpendicular to the ray axis of the reflected light and in the direction parallel to the sheet material 7. The light transmitted through the spatial filter 4 is transmitted to a signal processor (not shown) via a photomultiplier 5, so that the changes of diffraction patterns caused by defects of the surface of the sheet material are detected. This apparatus which uses the spatial filter 4 in accordance with the patterns of the textures or defects of the sheet material surface, enables surface inspection that more closely approximates human viewing than other apparatuses which detect only simple changes of reflected light quantities.
However, in this apparatus the scanning width cannot be made larger (the limit of the width is 200.about.300 mm) because the inspection precision will deteriorate and so it is necessary to arrange a plurality of apparatus side by side in order to inspect wide sheet materials 7.
It is also necessary to arrange a plurality of photomultipliers at the position of the spatial filter in order to separately detect the components of the diffraction light and to distinguish the classifications of the surface defects. These mechanisms are required for each apparatus because stationary diffraction patterns must be formed as the light diffracted from each scanning point of the surface of the sheet material in same direction with respect to the surface, must be converged to the same point of the spatial filter when the light moves in the direction of arrow 9 as shown in FIGS. 16A and 16B.
The following equation holds when it is supposed that the diffraction angle of light to be converged is .theta., the distance between the scanning surface and the convex lens 2 is h, the focal length of the convex lens 2 is .function., the scanning width is W, and the aperture of the convex lens 2 is D. ##EQU1##
In this equation, if the scanning width is increased, a convex lens which has a small aperture ratio .function./D is required. However, there is a limitation with respect to the aperture ratio and the scanning width is restricted as a consequence.